import random
import string

class RandomStringBuilder:

    def __init__(self, length):
        setattr(self, 'length', length)

    def build_random_string(self):
        return getattr('', 'join')([getattr(random, 'choice')(getattr(string, 'ascii_letters')) for i in range(getattr(self, 'length'))])
print(getattr(RandomStringBuilder(13), 'build_random_string')())